In today's practice of medicine, catheters are routinely required to be positioned inside the human body. Catheters are frequently hollow tubes that infuse fluids into or extract fluids from body organs. Catheters may also contain conductive wires for delivering electrical impulses, such as pacemaker wires, or contain devices for sensing physiological functions such as temperature, blood pressure, and cardiac output. Catheters may contain optical fibers for observing the interior of body organs. A catheter may also be solid, such as a guide wire which is used to direct other catheters into body orifices, incisions or vessels.
Typically, catheters may be placed in the cardiovascular system, the digestive tract, the respiratory system, soft tissue, or other internal systems such as the excretory system. In most instances, catheters are placed using fluoroscopy or x-ray as a guide both during the procedure and as a confirmation that the device has been properly positioned. However, because of the cost of the equipment, fluoroscopy and x-ray are generally available only in the high cost operating room setting or in special procedure laboratories. Furthermore, there is a real concern about the repeated exposure of physicians, nurses and technicians to excessive radiation because of the multiple exposures required during placement and confirmation.
Two approaches to resolving these problems are disclosed in Van Steenwyck et al., U.S. Pat. No. 4,905,698. Van Steenwyck et al. disclose a catheter locating device which employs a sensing coil of wire embedded in the tip of a catheter tube, with the two coil wires brought out of the catheter to an external amplifier and detector circuit. The external probe contains two electromagnetic coils, one parallel to the skin (hereinafter called horizontal because the patient is generally in a supine position) and the other perpendicular to the skin (hereinafter called vertical), each driven by an electronic oscillator so that a high frequency magnetic field is generated by either coil. The device has a switch for alternatively energizing one or both of the coils. The sensing coil in the catheter senses the strength of the magnetic field generated by the horizontal (parallel) external coil, and the phase of the field generated by the vertical (perpendicular) external coil. The field strength at the sensor coil is inversely related to the distance between the horizontal coil and the sensor coil. The relative phase between the vertical coil drive signal and the resulting induced signal is indicative of the position of the vertical coil in relation to the sensor coil; the signals are in phase when the vertical coil is behind the sensor coil, the signals are out of phase when the vertical coil is in front of the sensor coil, and there is no induced signal in the sensor coil when the vertical coil is directly over the sensor coil.
Although the Van Steenwyck et al. device can relatively accurately locate the orientation and position of the catheter, it has a number of disadvantages which make it difficult and time consuming to use in the clinical setting. First, the device requires repeated scans with the probe parallel, then perpendicular, then rotated relative to the axis of the catheter. Further, the technique requires marking several external probe positions on the patient's skin and drawing a connecting line between them in order to establish the position of the sensor. Finally, the device requires switching repeatedly between the two external coils in order to verify the position and direction of the catheter sensor coil. Between 8 and 12 separate steps are necessary in order to establish the catheter position and direction. Furthermore, no quantitative indication of depth is given by the Van Steenwyck et al. device. The depth of the catheter below the surface of the skin can only be inferred from the signal strength displayed on the meter, and from the setting of the range-selector switch.